inparticle physics,antimatteris the extension of the concept of theantiparticletomatter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example, apositron(the antiparticle of the electron ore+) and anantiproton(p) can form anantihydrogenatom in the same way that an electron and a proton form anormal matterhydrogen atom. Furthermore, mixing matter and antimatter can lead to theannihilationof both in the same way that mixing antiparticles and particles does, thus giving rise to high-energyphotons(gamma rays) or other particle–antiparticle pairs.
There is considerable speculation as to why the observable universe is apparently almost entirely matter, whether there exist other places that are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed. At this time, the apparentasymmetry of matter and antimatterin thevisible universeis one of the greatestunsolved problems in physics. The process by which this asymmetry between particles and antiparticles developed is calledbaryogenesis.
History of the concept
Negative matterhas appeared in the past in several, now abandoned, theories of matter. Using the once popularvortex theory of gravitythe possibility of matter with negative gravity was discussed byWilliam Hicksin the 1880s. In either the 1880s or 1890s)Karl Pearsonproposed the existence of "squirts" (sources) and sinks of the flow ofaether. The squirts represented normal matter and the sinks represented negative matter, a term which Pearson is credited with coining. Pearson's theory required a fourth dimension for the aether to flow from and into.[1]
The term antimatter was first used byArthur Schusterin two rather whimsical letters toNaturein 1898,[2]in which he coined the term. He hypothesizedantiatoms, whole antimatter solar systems and discussed the possibility of matter and antimatter annihilating each other. Schuster's ideas were not a serious theoretical proposal, merely speculation, and like the previous ideas, differed from the modern concept of antimatter in that it possessednegative gravity.[3]
The modern theory of antimatter begins in 1928, with a paper[4]byPaul Dirac. Dirac realised that hisrelativistic versionof theSchrödinger wave equationfor electrons predicted the possibility of antielectrons. These were discovered byCarl D. Andersonin 1932 and namedpositrons(a contraction of "positive electrons"). Although Dirac did not himself use the term antimatter, its use follows on naturally enough from antielectrons, antiprotons etc.[5]A completeperiodic tableof antimatter was envisaged byCharles Janetin 1929.[6]
[edit]Notation
One way to denote an antiparticle is by adding a bar over the particle's symbol. For example, the proton and antiproton are denoted aspandp, respectively. The same rule applies if one were to address a particle by its constituent components. A proton is made up ofuudquarks, so an antiproton must therefore be formed fromuudantiquarks. Another convention is to distinguish particles by theirelectric charge. Thus, the electron and positron are denoted simply ase−ande+respectively. However, to prevent confusion, the two conventions are never mixed.
[edit]Origin and asymmetry
Almost all matter observable from the Earth seems to be made of matter rather than antimatter. Many scientists believe that this preponderance of matter over antimatter (known asbaryon asymmetry) is the result of an imbalance in the production of matter and antimatter particles in the early universe, in a process calledbaryogenesis. If antimatter-dominated regions of space existed, the gamma rays produced in annihilation reactions along the boundary between matter and antimatter regions would be detectable. The amount of matter presently observable in the universe only requires an imbalance in the early universe on the order of one extra matter particle per billion matter-antimatter particle pairs.[7]
Antiparticles are created everywhere in theuniversewhere high-energy particle collisions take place. High-energycosmic raysimpactingEarth's atmosphere(or any other matter in thesolar system) produce minute quantities of antiparticles in the resultingparticle jets, which are immediately annihilated by contact with nearby matter. They may similarly be produced in regions like thecenterof theMilky Wayand other galaxies, where very energetic celestial events occur (principally the interaction ofrelativistic jetswith theinterstellar medium). The presence of the resulting antimatter is detectable by the twogamma raysproduced every timepositronsannihilate with nearby matter. Thefrequencyandwavelengthof the gamma rays indicate that each carries 511keVof energy (i.e. therest massof anelectronmultiplied byc2).
Recent observations by theEuropean Space Agency'sINTEGRAL satellitemay explain the origin of a giant cloud of antimatter surrounding the galactic center. The observations show that the cloud is asymmetrical and matches the pattern ofX-ray binaries(binary star systems containing black holes or neutron stars), mostly on one side of the galactic center. While the mechanism is not fully understood, it is likely to involve the production of electron–positron pairs, as ordinary matter gains tremendous energy while falling into astellar remnant.[8][9]
Antimatter may exist in relatively large amounts in far away galaxies due tocosmic inflationin the primordial time of the universe.NASAis trying to determine if this is true by looking for X-ray and gamma-ray signatures of annihilation events incollidingsuperclusters.[10]
[edit]Artificial production
Antiparticles are also produced in any environment with a sufficiently high temperature (mean particle energy greater than thepair productionthreshold). During the period ofbaryogenesis, when the universe was extremely hot and dense, matter and antimatter were continually produced and annihilated. The presence of remaining matter, and absence of detectable remaining antimatter,[11]also calledbaryon asymmetry, is attributed toviolationof theCP-symmetryrelating matter and antimatter. The exact mechanism of this violation during baryogenesis remains a mystery.
Positrons can also be produced by radioactiveβ+decay, but this mechanism can occur both naturally and artificially.
[edit]Positrons
Main article:Positron
Positrons were reported[12]in November 2008 to have been generated byLawrence Livermore National Laboratoryin larger numbers than by any previous synthetic process. Alaserdroveelectronsthrough a millimeter radiusgoldtarget'snuclei, which caused the incoming electrons to emitenergyquanta, that decayed into both matter and antimatter. Positrons were detected at a higher rate and in greater density than ever previously detected in a laboratory. Previous experiments made smaller quantities of positrons using lasers and paper-thin targets; however, new simulations showed that short, ultra-intense lasers and millimeter-thick gold are a far more effective source.[13]
[edit]Antiprotons, antineutrons, and antinuclei
Main articles:AntiprotonandAntineutron
The antiproton was experimentally confirmed in 1955 byUniversity of California, BerkeleyphysicistsEmilio SegrèandOwen Chamberlain, for which they were awarded the 1959Nobel Prize in Physics[14]. An antiproton consists of two upantiquarkand one down antiquark (uud). The properties of the antiproton that have been measured all match the corresponding properties of the proton, with the exception that the antiproton has opposite electric charge and magnetic moment than the proton. Shortly afterwards, in 1956, the antineutron was discovered inproton–proton collisions at theBevatron(Lawrence Berkeley National Laboratory) byBruce Corkand colleagues.[15]
In addition to antibaryons, anti-nuclei consisting of multiple bound antiprotons and antineutrons have been created. These are typically produced at energies far too high to form antimatter atoms (with bound positrons in place of electrons). In 1965, a group of researchers led byAntonino Zichichireported production of nuclei of antideuterium at the Proton Synchrotron atCERN.[16]At roughly the same time, observations of antideuterium nuclei were reported by a group of American physicists at the Alternating Gradient Synchrotron atBrookhaven National Laboratory.[17]
[edit]Antihydrogen atoms
Main article:Antihydrogen
In 1995CERNannounced that it had successfully brought into existence nine antihydrogen atoms by implementing theSLAC/Fermilabconcept during thePS210 experiment. The experiment was performed using theLow Energy Antiproton Ring(LEAR), and was led by Walter Oelert and Mario Macri. Fermilab soon confirmed theCERNfindings by producing approximately 100 antihydrogen atoms at their facilities. The antihydrogen atoms created during PS210, and subsequent experiments (at bothCERNand Fermilab) were extremely energetic ("hot") and were not well suited to study. To resolve this hurdle, and to gain a better understanding of antihydrogen, two collaborations were formed in the late 1990s —ATHENAandATRAP. In 2005, ATHENA disbanded and some of the former members (along with others) formed theALPHA Collaboration, which is also situated at CERN. The primary goal of these collaborations is the creation of less energetic ("cold") antihydrogen, better suited to study.
In 1999CERNactivated theAntiproton Decelerator(AD), a device capable of decelerating antiprotons from3.5GeVto5.3MeV— still too "hot" to produce study-effective antihydrogen, but a huge leap forward. In late 2002 the ATHENA project announced that they had created the world's first "cold" antihydrogen.[18]The ATRAP project released similar results very shortly thereafter.[19]The antiprotons used in these experiment were cooled by decelerating them with the AD, passing them through a thin sheet of foil, and finally capturing them in a Penning-Malmberg trap.[20]The overall cooling process is effective, but highly inefficient; approximately 25 million antiprotons leave the AD and roughly 25,000 make it to the Penning-Malmberg trap, which is about1⁄1000or 0.1% of the original amount.
The antiprotons are still hot when initially trapped. To cool them further, they are mixed into an electron plasma. The electrons in this plasma cool via cyclotron radiation, and then sympathetically cool the antiprotons viaCoulombcollisions. Eventually, the electrons are removed by the application of short-duration electric fields, leaving the antiprotons with energies less than 100meV.[21]While the antiprotons are being cooled in the first trap, a small cloud of positrons is captured fromradioactivesodiumin a Surko-style positron accumulator,[22]This cloud is then recaptured in a second trap near the antiprotons. Manipulations of the trap electrodes then tip the antiprotons into the positron plasmas, where some combine with antiprotons to form antihydrogen. This neutral antihydrogen is unaffected by the electric and magnetic fields used to trap the charged positrons and antiprotons, and within a few microseconds the antihydrogen hits the trap walls, where it annihilates. Some hundreds of millions of antihydrogen atoms have been made in this fashion.
Most precision tests of the properties of antihydrogen can only be done if the antihydrogen were trapped; i.e. held in place for a long time. While antihydrogen atoms are electrically neutral, their spin producesmagnetic moments. These magnetic moments will interact with an inhomogeneous magnetic field; some of the antihydrogen atoms will be attracted to a magnetic minimum. Such a minimum can be created by a combination of mirror and multipole fields.[23]Antihydrogen can be trapped in such a magnetic minimum (minimum-B) trap; in November 2010, the ALPHA collaboration announced that they had so trapped 38 antihydrogen atoms for about a sixth of a second.[24]This was the first time that neutral antimatter had been trapped.
The biggest limiting factor in the large scale production of antimatter is the availability of antiprotons. Recent data released byCERNstates that, when fully operational, their facilities are capable of producing107antiprotons per minute.[25]Assuming an 100% conversion of antiprotons to antihydrogen, it would take 100 billion years to produce 1gramor 1moleof antihydrogen (approximately6.02×1023atoms of antihydrogen).